I. Technical Field
The present invention relates to a cooling apparatus, more specifically a compressor of a refrigeration cycle driven by an electric motor of the speed control type.
II. Description of the Related Art
JP-H11-173729A and JP-S60-188775A disclose cooling apparatuses of refrigerators. It is known that an inner temperature, which is a temperature inside a cooling apparatus, is detected and a rotating speed of the electric motor is controlled using the detected inner temperature. In this technique, the rotating speed of the motor is controlled so as to be increased as the inner temperature rises, exceeding a set temperature, for example, about 5° C., so that a cooling capacity of the refrigeration cycle is increased, whereupon the inner temperature is kept substantially equal to the set temperature.
In the above-described conventional cooling apparatuses, however, when the inner temperature which is close to a room temperature (which is a temperature in a room where the cooling apparatus is installed), is reduced to a set temperature (such as at the time of installation of a refrigerator), the rotating speed of the motor is increased extremely since quick cooling is carried out at an initial stage under the condition where the difference between a set temperature (4° C., for example) and the inner temperature (15° C., for example) is considerably large. In this state, a large current flows into the motor. In order to allow a large current to flow, an electric motor with a large capacity and a power supply circuit with a large capacity are needed, thereby increasing the sizes of the motor and the power supply circuit. As a result, production costs of the motor and power supply circuit are increased.
In view of the foregoing problem, the conventional art has developed a technique for reducing the capacities of the motor and power supply circuit by utilizing the upper limit of the rotating speed of the motor.
However, the refrigeration cycle of the aforementioned type has a characteristic that the input current is rendered lower at the same rotating speed when a temperature is low in a room where, for example, a refrigerator is installed, than when the temperature is high in the room. However, the rotating speed of the motor is provided with a uniform upper limit in the conventional control manner. As a result, for example, the input current is sufficiently small when the temperature in a room is low. Despite this, there is a case where the rotating speed of the motor reaches an upper limit thereof such that the cooling apparatus cannot deliver its original full cooling capacity. In this case, a useless rotating speed limit is rendered effective so that a time period required to reach a set temperature is prolonged. This means that the refrigerating capacity cannot be utilized effectively.
Thus, there is a need in the art for a cooling apparatus which can deliver an original refrigerating capacity while capacities of the motor and the power supply circuit can be reduced.